Water

Water has the molecular formula $\text{H}_2\text{O}$. The two O—H bonds are at an angle of about 104.5°, giving the molecule its familiar V shape.

Polarity. Oxygen is more electronegative than hydrogen — its nucleus attracts the bonding electrons more strongly. The shared electrons spend more time near the O than the H atoms, leaving:

• a small negative charge ($\delta^-$) on the O
• a small positive charge ($\delta^+$) on each H

Water is therefore a polar molecule or dipole.

Hydrogen bonding. The $\delta^+$ H of one water molecule is electrostatically attracted to the $\delta^-$ O of a neighbouring water molecule. This weak intermolecular attraction is the hydrogen bond.

Each water molecule can form up to four hydrogen bonds — two through its two O—H hydrogens (acting as donor) and two through the two lone pairs on its oxygen (acting as acceptor). At room temperature these bonds form, break and re-form continuously, lasting only a few picoseconds each. Yet at any instant, the average water molecule is hydrogen-bonded to about 3.4 others, and this large network of weak bonds is responsible for almost every unusual property of water.

Because water is polar, it interacts electrostatically with any species that carries a charge (ions) or has its own dipole (other polar molecules). When such a solute is placed in water, water molecules surround each particle with their oppositely-charged ends — building a hydration shell:

• $\delta^+$ H atoms face the anion (or negative end of a dipole).
• $\delta^-$ O atoms face the cation (or positive end).

This separation overcomes the attraction between the original ions or molecules and disperses them as a true solution.

What water dissolves.

• Ions — Na⁺, K⁺, Ca²⁺, Cl⁻, NO₃⁻ and the many other mineral nutrients of plants and animals.
• Polar organic molecules — glucose, fructose, amino acids, sugars.
• Respiratory gases — O₂ and CO₂ (though only slightly soluble, enough to support diffusion).

Biological consequences.

• Transport medium. Blood plasma, lymph, xylem sap, phloem sap, urine and cytoplasm are all aqueous solutions. Water carries solutes around the organism.
• Reaction medium. Most metabolic reactions occur in aqueous solution. Enzymes can only act on dissolved substrates, and the products of digestion are dissolved before absorption.
• Excretion. Soluble nitrogenous waste (urea, ammonia) is dissolved and removed in urine.

What water does not dissolve. Non-polar substances (lipids, hydrocarbons) are hydrophobic and do not dissolve. This property is exploited by the lipid bilayer of membranes, which uses water's repulsion of non-polar tails to maintain the structure of the cell.

The specific heat capacity of a substance is the energy needed to raise the temperature of 1 kg of it by 1 K. For water it is about 4.18 kJ kg⁻¹ K⁻¹ — much higher than most other liquids.

Why? When energy is added to water, the energy first goes into breaking hydrogen bonds. Only after a hydrogen bond has been broken can the kinetic energy of the molecule rise — and only kinetic energy is detected as temperature. Because water has so many hydrogen bonds, a large amount of energy must be absorbed before the temperature noticeably rises.

Biological consequences.

• Aquatic habitats — oceans, lakes and ponds change temperature only slowly through the day and through the seasons. Aquatic organisms experience a relatively stable thermal environment.
• Cytoplasm and body fluids — mostly water, they resist temperature change. Enzymes therefore stay near their optimum temperature in spite of variations in environmental temperature, especially in larger organisms with a high water content.
• Climate buffering — oceans store enormous quantities of thermal energy and moderate the climates of coastal regions.

The latent heat of vaporisation is the energy needed to convert 1 kg of a liquid into vapour without changing its temperature. For water this is about 2260 kJ kg⁻¹ — exceptionally high because almost all of the hydrogen bonds must be broken to free a molecule into the gas phase.

Biological consequences.

• Sweating — mammals lose heat by evaporating sweat from the skin. A small mass of water carries a large amount of heat away with it, an efficient cooling mechanism in warm climates.
• Transpiration cooling — evaporation of water from spongy mesophyll surfaces inside the leaf draws heat from the leaf, helping to prevent overheating in strong sunlight.
• Panting in dogs and other mammals exploits the same principle on a moist tongue surface.

The latent heat of fusion is the energy required to melt 1 kg of solid (or released when 1 kg of liquid freezes). For water this is also high (~334 kJ kg⁻¹) — bodies of water release substantial heat to their surroundings as they freeze, slowing the rate of further temperature drop and again buffering aquatic environments against cold.

Cohesion is the attraction between molecules of the same substance — between water molecules through hydrogen bonds. Adhesion is the attraction between water and another (polar) substance.

Cohesion-tension theory of transpiration.

1. Transpiration from the leaf creates negative pressure at the top of the xylem.
2. The continuous water column in xylem vessels is held together by cohesion — hydrogen bonds between adjacent water molecules.
3. The column does not break under tension because of these cohesive forces.
4. Adhesion of water to the lignified walls of xylem vessels also helps maintain the column.
5. Water is pulled all the way up — even in 100 m tall trees — without snapping.

Surface tension. At the air-water interface, water molecules at the surface have no neighbours above them, so all their cohesive forces pull inward and sideways. The surface behaves like a thin elastic film. Surface tension supports small animals such as pondskaters and provides the underside of the surface on which mosquito larvae hang.

Most substances become more dense when they freeze, because the molecules pack closer together. Water does the opposite.

In liquid water, hydrogen bonds form, break and re-form continuously, allowing molecules to slip close to each other. As water cools below 4 °C the molecules slow down and the hydrogen bonds last longer; below 0 °C they lock into a regular hexagonal lattice in which each water is hydrogen-bonded to four others at fixed angles.

The hexagonal lattice contains more empty space than liquid water, so ice has a lower density (~9 % less dense than liquid water). Ice therefore floats.

Biological consequences.

• A layer of ice on the surface of a lake or pond insulates the liquid water beneath. The body of water remains at ~4 °C, its temperature of maximum density.
• Aquatic organisms — fish, invertebrates, plants, microbes — survive winter in the unfrozen water.
• Without this anomaly, water bodies would freeze from the bottom upward, and most temperate aquatic ecosystems could not persist through cold seasons.